1. Field of the Invention
The present invention relates to a DC fan and, more particularly, to a DC fan of an inner rotor type.
2. Description of the Related Art
FIG. 1 shows a conventional DC fan 9 of an inner rotor type. Specifically, the DC fan 9 includes a housing 91 having a compartment 910. A stator 92 is fixed to an inner periphery of the housing 91. A rotor 93 is rotatably received in the compartment 910. An impeller 94 is coupled to an end of the rotor 93. The stator 92 includes an annular permanent magnet 921 surrounding the rotor 93. The stator 92 further includes a brush 922 adjacent an end of the permanent magnet 921 and electrically connected to a DC power source. The rotor 93 includes a shaft 931, a winding core 932, and a converter 933. An end of the shaft 931 extends beyond the housing 91. The winding core 932 is mounted around the shaft 931 and includes an outer periphery facing the permanent magnet 921 with an air gap formed between the winding core 932 and the permanent magnet 921. Electricity passing through the coil of the winding core 932 interacts with the magnetic field created by the permanent magnet 921. The converter 933 is also mounted around the shaft 931 and electrically connected to the coil of the winding core 932. The converter 933 has an outer periphery for sliding, electrical contact with the brush 922.
In operation, when DC power is supplied from the DC power source to the brush 922 of the stator 92, the DC power is transmitted through the converter 933 of the rotor 93 to the coil of the winding core 932. Electric current generated in the coil by the DC power interacts with a magnetic field created by the permanent magnet 921, driving the rotor 93 to rotate relative to the stator 92. The speed of the rotor 93 can be decided by controlling the DC power to modulate the current in the coil of the winding core 932.
However, the converter 933 is parallel to the shaft 931 and includes a plurality of spaced converter plates, such that sparks are liable to occur between the brush 922 and the converter 933 when the brush 922 moves from one of the converter plates to an adjacent converter plate. At the same time, a noise signal adversely affecting the system self-control is apt to be generated. Further, the brush 922 must be in tight contact with the outer periphery of the converter 933 to assure electrical connection therebetween. To avoid adverse affect to the electrical connection between the brush 922 and the converter 933 by accumulated carbon resulting from sparks, the worn-out brush 922 must be periodically replaced, and the outer periphery of the converter 933 must be periodically cleaned. Further, although the speed of the rotor 93 can be decided by controlling the DC power, noise signals are apt to be generated during DC power transmission between the brush 922 and the converter 933, for the brush 922 moves between the converter plates. As a result, the DC fan 9 can not be utilized in products requiring precise control of the speed of the fan. Further, when the load of the DC fan 9 is changed, an additional sensing device such as a mechanical governor, an electronic governor, a speed signal generator, or an optical encoder is required to feed back the actual speed of the DC fan 9, so that the voltage of the DC power can be adjusted to drive the DC fan 9 to rotate at an expected speed. However, the manufacturing costs of the DC fan 9 are increased, while the accuracy of the sensing device affects the controlling quality of the speed.
Thus, a need exists for a DC fan allowing precise speed control while allowing easy assembly and manufacture at low costs.